Generally, as shown in FIG. 1, a door inner panel 1 for a vehicle has a plurality of burring holes 1a which are used as fixing holes to install a variety of parts such as a door trim, a window regulator, etc.
A burring hole is a through hole which has a conical depression in a perimeter thereof so that a head of a bolt, a screw, or the like inserted into the hole can be prevented from protruding outward. In other to form such a burring hole, in conventional techniques, burring hole forming apparatus and method as shown in FIG. 2 have been used.
Hereinafter, conventional burring hole forming apparatus and method will be described in brief.
First, a conical die 2 is brought into contact with a target portion of an inner surface of a door inner panel 1 which is to be machined. A vertical part of a jig base 3 including the die 2 is brought into contact with the entirety of the inner surface of the door inner panel 1. Then, the target portion of the door inner panel 1 is punched from an outer surface of the door inner panel 1 with a burring punch 6 that is mounted to a jig slide 5, which converts vertical force of a jig driver 4 into horizontal force (or a jig pad may be coupled to the jig slide and the burring punch may be mounted to the jig pad). In this way, a burring hole 1a is formed in the door inner panel 1.
In the conventional techniques, a double jig tool for machining a door inner panel has been mainly used as a representative example of the above-mentioned burring hole forming apparatus. As shown in FIG. 3, in a conventional double jig tool for machining a door inner panel, a jig base 3 is linearly operated when a jig driver 4 moves downward. Subsequently, when the jig driver 4 further moves downward, a jig slide 5 is operated in the direction opposite to a direction in which the jig base 3 moves. A jig pad mounted to the jig slide 5 (and a burring punch mounted to the jig pad) moves toward the door inner panel 1. Then, the jig pad (and the burring punch mounted to the jig pad) comes into contact with the door inner panel 1 and machines it, thus forming a burring hole. However, in the case of the conventional double jig tool having the above-mentioned configuration, as shown in FIG. 4, any structure cannot be installed on a rear surface of the vertical part of the jig base 3, and therefore, durability of the jig base 3 is low. Thereby, the jig base 3 may be frequently damaged during repetitive burring hole forming operations.
Furthermore, the conventional technique is problematic in that because the size of the jig base 3 is comparatively large, space required to install the jig base 3 is also large.
Moreover, in the conventional technique, a large number of parts are required to embody the linear movement of the jig base 3.